With another person helping, turn off all RV water supply, disconnect the supply hose from the toilet, have a towel ready, have your helper momentarily turn on the water pressure to see if you get fresh water flow (or if the fresh water line is clogged). If the line is clogged, you may have to also disconnect the fresh water main shore supply line at the inside of the UDC, then blow the line out from the inside to the supply end with compressed air.
This is a chance to install a 1/2 inch Sharkbite (brand) PEX shutoff valve on your toilet freshwater supply line. It will come in handy in the future.